Why Am I Always Tired? The 8 Most Common Causes and What to Do About Each One
Fatigue that does not improve with sleep is almost never a sleep problem. It is a nutritional or metabolic problem. Iron deficiency affects over 30% of women globally. Vitamin D deficiency affects 42% of Americans. Here is how to identify which cause is yours.
Persistent fatigue that sleep does not fix has a nutritional or metabolic cause in the vast majority of cases. The four most common are iron deficiency, vitamin D deficiency, B12 deficiency, and hypothyroidism — all of which are identifiable through a routine blood test panel and addressable with targeted intervention. Calorie restriction, dehydration, magnesium deficiency, and blood sugar dysregulation complete the eight most common causes. Most people have more than one simultaneously.
Fatigue is one of the most common presenting complaints in primary care — it accounts for 10 to 20% of all GP visits, according to StatPearls. The vast majority of these visits involve fatigue that has persisted for weeks or months despite what feels like adequate sleep. The person is tired but not sleepy. They sleep the same number of hours but wake unrefreshed. Energy levels do not respond to caffeine the way they used to. Concentration is lower. Motivation is harder to find.
This pattern — persistent fatigue that is not resolved by more sleep — is the hallmark of a nutritional or metabolic cause rather than a sleep disorder. The causes below are ranked roughly by global prevalence.
Women globally affected by iron deficiency anaemia — the leading nutritional cause of chronic fatigue worldwide. In some populations the prevalence is over 50%.
World Health Organization, Global Anaemia DataAmericans estimated to have vitamin D deficiency or insufficiency — strongly linked to fatigue, muscle weakness, and mood disorders. Most people have no symptoms beyond tiredness.
National Institutes of Health, Vitamin D Fact SheetAdults with an undiagnosed thyroid disorder, according to Harvard Health Publishing. The thyroid controls metabolic rate — even mild dysfunction produces significant fatigue and weight changes.
Harvard Health PublishingThe 8 Most Common Causes of Persistent Fatigue
Iron is essential for haemoglobin production — the protein in red blood cells that carries oxygen from the lungs to every tissue in the body. When iron stores are low, fewer red blood cells are produced and those that are produced carry less oxygen. Every cell in the body receives less fuel and the result is profound fatigue, shortness of breath on exertion, poor concentration, and reduced physical and mental performance.
The critical distinction is between serum iron and ferritin. Serum iron measures the iron currently circulating in the blood. Ferritin measures iron stored in tissues. A person can have a normal serum iron level and severely depleted ferritin stores — meaning they are iron deficient without yet being anaemic. Fatigue from low ferritin is real and significant even when haemoglobin remains within the normal range. This is why a full iron panel including ferritin is essential rather than just a standard full blood count.
Iron deficiency disproportionately affects women of reproductive age due to monthly blood loss, pregnant women, people who eat plant-based diets (plant-source iron is significantly less bioavailable than haem iron from animal sources), and anyone in a sustained calorie deficit where total food volume and nutrient density are reduced.
Vitamin D is technically a hormone rather than a vitamin — its receptor is expressed in nearly every tissue in the body including the brain, muscle, and immune system. Its roles extend far beyond bone health. Vitamin D is involved in mitochondrial function, the cellular energy-production process. It modulates serotonin synthesis which affects mood and motivation. It supports muscle protein synthesis and physical performance. Deficiency across all of these pathways produces a distinctive fatigue that feels different from iron deficiency — more like a generalised lack of motivation, muscle heaviness, and low mood than the breathless, heart-pounding exhaustion of anaemia.
Vitamin D deficiency is extraordinarily common because the primary source is skin synthesis from UVB sunlight rather than food. In northern latitudes, indoor lifestyles, dark skin pigmentation (which requires longer sun exposure for equivalent synthesis), and consistent sunscreen use all significantly reduce synthesis. Food sources are limited — oily fish, eggs, and fortified dairy provide modest amounts but are insufficient to maintain optimal levels without sun exposure or supplementation.
Vitamin B12 is essential for red blood cell production, neurological function, and DNA synthesis. Deficiency produces a specific form of anaemia called megaloblastic anaemia — red blood cells are larger than normal but fewer in number and carry oxygen less efficiently. The fatigue from B12 deficiency is often accompanied by neurological symptoms including tingling or numbness in the hands and feet, difficulty with balance, and cognitive slowing — symptoms that distinguish it from simple iron deficiency anaemia.
B12 deficiency develops slowly and silently. The body stores enough B12 in the liver to last 3 to 5 years, which means by the time symptoms appear, stores are substantially depleted. Risk factors include a plant-based diet (B12 is found almost exclusively in animal products), age over 50 (gastric acid production declines with age, reducing B12 absorption), use of metformin (which reduces B12 absorption in the gut), and long-term use of proton pump inhibitors. People on GLP-1 medications who are also taking metformin are at elevated risk.
The thyroid gland regulates metabolic rate throughout the body. When it underproduces thyroid hormones, everything slows — energy production, digestion, heart rate, body temperature regulation, and cognitive function. The fatigue of hypothyroidism is distinctive: it is associated with weight gain despite unchanged diet, feeling cold when others are comfortable, dry skin and hair, constipation, and a general slowing that feels like the body is running at reduced capacity.
Subclinical hypothyroidism is particularly underdiagnosed. TSH levels are slightly elevated but still within the broad “normal” range, yet symptoms are present and meaningful. Many people with subclinical hypothyroidism report significant fatigue improvement with treatment even when their TSH is not dramatically elevated. Hashimoto’s thyroiditis — an autoimmune condition — is the most common cause of hypothyroidism and requires thyroid antibody testing for diagnosis. Standard TSH testing alone may miss early Hashimoto’s.
When calorie intake falls significantly below maintenance requirements — whether through deliberate dieting or medication-induced appetite suppression — the body activates a survival mechanism called adaptive thermogenesis. Metabolic rate falls by more than would be predicted from weight loss alone. Thyroid hormones down-regulate. Cortisol rises. The body prioritises fat storage and reduces energy expenditure across all systems.
The result is profound fatigue that has a specific character: it is accompanied by feeling cold, reduced motivation, brain fog, and a weight loss plateau. People often interpret this as the diet “working” — they are eating very little and losing weight. In reality, the metabolic slowdown from very low intake is undermining both fat loss efficiency and energy levels simultaneously. Eating too little is one of the most underappreciated causes of fatigue in people actively trying to lose weight.
For people on GLP-1 medications specifically, appetite suppression can inadvertently push daily intake below 800 to 900 calories without the person realising it. The not eating enough on GLP-1 guide and the adaptive thermogenesis guide cover this in full.
Even mild dehydration of 1 to 2% of body weight measurably reduces physical and cognitive performance. A 2011 study published in the Journal of Nutrition found that women with just 1.4% dehydration reported significantly higher fatigue scores, poorer concentration, and more frequent headaches. Dehydration reduces blood volume, forcing the heart to work harder to deliver oxygen to tissues. The brain is particularly sensitive to hydration status — cognitive fatigue from dehydration often precedes physical fatigue.
Electrolyte imbalance compounds dehydration. Sodium, potassium, and magnesium are all required for cellular energy production, nerve transmission, and muscle function. They are primarily consumed through food. When food intake drops significantly — as it does on most calorie-restricted diets and dramatically on GLP-1 medications — dietary electrolyte intake drops with it. Drinking more water without replacing electrolytes can actually worsen the imbalance by diluting what remains. Muscle cramps, dizziness, and headaches alongside fatigue are the classic signs of electrolyte depletion.
Magnesium is required for over 300 enzymatic reactions in the body, including virtually every step of ATP production — the energy currency of the cell. It is essential for glucose metabolism, protein synthesis, nerve function, and muscle contraction. When magnesium is deficient, energy production at the cellular level is impaired. The fatigue is often accompanied by muscle weakness, cramps, poor sleep quality, and difficulty managing stress.
Magnesium deficiency is extremely common in Western populations and significantly underdiagnosed because serum magnesium — the standard blood test — only reflects 1% of total body magnesium. A person can have severely depleted cellular and tissue magnesium stores while showing a normal serum level. Red blood cell magnesium is a more accurate test but rarely ordered in routine panels. The most practical approach is empirical — if fatigue, muscle cramps, and poor sleep are present with low dietary magnesium intake, supplementation is low-risk and often produces rapid improvement.
The brain runs almost exclusively on glucose. When blood sugar fluctuates significantly — spiking after high-carbohydrate meals then crashing 2 to 3 hours later — energy levels follow the same pattern. The post-lunch energy crash that many people attribute to a large meal is often reactive hypoglycaemia — the blood sugar drop that follows an excessive insulin response to a high-glycaemic load.
Insulin resistance produces a more chronic pattern: glucose circulates in the blood but cannot enter cells efficiently, leaving cells energy-starved despite adequate food intake. The fatigue of insulin resistance is often accompanied by difficulty losing weight, strong carbohydrate cravings, brain fog after eating, and increased hunger despite sufficient calories. HbA1c and fasting blood glucose in a routine blood panel will identify blood sugar dysregulation that may be contributing to fatigue years before a diabetes diagnosis is reached.
Dietary strategies that stabilise blood sugar — eating protein first at every meal, reducing refined carbohydrates, increasing fibre, and avoiding high-glycaemic foods on their own — produce consistent improvements in energy stability. The same nutritional strategies that support the Fueled Framework metabolic system directly address this cause.
The Blood Tests to Ask For
A comprehensive fatigue panel costs very little and identifies the majority of common causes in a single appointment. Most GPs will order this panel when fatigue is the presenting complaint. Ask specifically for the following:
Write down when the fatigue started, how many hours of sleep you average, whether it has got progressively worse, what other symptoms accompany it (cold intolerance, hair loss, muscle cramps, brain fog, weight changes), and any medications you take. This information helps your GP order the most targeted panel and avoid the “everything is normal” result that comes from a basic panel that misses ferritin and vitamin D.
All 8 Causes at a Glance
| Cause | Key symptoms beyond fatigue | Blood test | Primary intervention |
|---|---|---|---|
| Iron deficiency | Breathlessness, pale skin, fast heartbeat | Ferritin, CBC | Dietary iron or supplementation |
| Vitamin D deficiency | Muscle weakness, low mood, frequent illness | 25-OH Vitamin D | 1,000–2,000 IU daily supplement |
| Vitamin B12 deficiency | Tingling extremities, cognitive slowing, anaemia | Serum B12, MMA | B12 supplement, sublingual or injection |
| Hypothyroidism | Weight gain, cold intolerance, dry hair, constipation | TSH, Free T4, TPO antibodies | Medical treatment (levothyroxine) |
| Calorie restriction / adaptive thermogenesis | Feeling cold, brain fog, weight plateau | Track intake | Increase calories above metabolic floor |
| Dehydration and electrolytes | Headaches, dizziness, muscle cramps | Clinical assessment | 2–2.5 litres water plus electrolytes daily |
| Magnesium deficiency | Muscle cramps, poor sleep, stress sensitivity | Serum magnesium (imperfect) | 200–400mg magnesium glycinate daily |
| Blood sugar dysregulation | Energy crashes after meals, carb cravings, brain fog | Fasting glucose, HbA1c | Protein-first meals, reduce refined carbs |
The energy and metabolic system
Persistent fatigue during fat loss is one of the most common and most fixable problems in nutrition. The Energy and Hydration hub covers the full framework. The adaptive thermogenesis guide explains why eating too little worsens fatigue rather than improving it. The magnesium and weight loss guide covers the specific role of magnesium in metabolic function. The GLP-1 fatigue guide covers the specific fatigue pattern for people on Ozempic, Wegovy, Mounjaro, or Zepbound.
Frequently Asked Questions
Fatigue that does not improve with sleep is almost always a nutritional or metabolic cause rather than a sleep problem. The most common causes are iron deficiency anaemia, vitamin D deficiency, B12 deficiency, hypothyroidism, calorie restriction triggering adaptive thermogenesis, dehydration, magnesium deficiency, and blood sugar dysregulation. Most of these are identifiable through a standard blood panel and addressable with targeted nutritional intervention.
The most common nutritional deficiencies causing chronic fatigue are iron deficiency (the leading cause globally — affects over 30% of women), vitamin D deficiency (affects 42% of Americans), vitamin B12 deficiency (particularly in plant-based eaters, over 50s, and metformin users), and magnesium deficiency (affects up to 50% of people in Western countries). These frequently occur together and can compound each other significantly.
Yes — significantly. When calorie intake falls below maintenance, the body activates adaptive thermogenesis which reduces metabolic rate, elevates cortisol, and down-regulates thyroid hormones. The result is fatigue, feeling cold, brain fog, and a weight loss plateau. This is particularly common on GLP-1 medications where appetite suppression can push intake below 800 to 900 calories without the person realising it. Increasing calorie intake above the metabolic floor typically resolves this type of fatigue within 1 to 2 weeks.
A comprehensive fatigue panel should include: full blood count, serum ferritin, TSH and Free T4, 25-OH vitamin D, vitamin B12, fasting blood glucose and HbA1c, and magnesium. Ask for ferritin specifically — standard full blood counts often omit it, and low ferritin causes significant fatigue even when haemoglobin is still within the normal range.
Yes. Even mild dehydration of 1 to 2% of body weight reduces cognitive performance, physical endurance, and subjective energy significantly. A 2011 Journal of Nutrition study found that 1.4% dehydration in women produced significantly increased fatigue, poorer concentration, and more frequent headaches. Dehydration is frequently overlooked because thirst signals are weak or absent, particularly in people on appetite-suppressing medications.
Research and References
- World Health Organization. Anaemia. Global prevalence data — over 30% of women globally affected by iron deficiency anaemia. who.int
- National Institutes of Health. Vitamin D — Fact Sheet for Health Professionals. Approximately 42% of Americans have vitamin D deficiency or insufficiency. ods.od.nih.gov
- Harvard Health Publishing. Thyroid disorders: 1 in 10 adults has undiagnosed thyroid dysfunction. health.harvard.edu
- Ganio MS, et al. Mild dehydration impairs cognitive performance and mood in men. British Journal of Nutrition. 2011. pubmed.ncbi.nlm.nih.gov
- Armstrong LE, et al. Mild dehydration affects mood in healthy young women. Journal of Nutrition. 2012;142(2):382–388. pubmed.ncbi.nlm.nih.gov
- Ankar A, Kumar A. Vitamin B12 Deficiency. StatPearls. Updated 2023. ncbi.nlm.nih.gov
- National Institutes of Health. Magnesium — Fact Sheet for Health Professionals. Magnesium required for over 300 enzymatic reactions including ATP production. ods.od.nih.gov
- Camaschella C. Iron-deficiency anaemia. New England Journal of Medicine. 2015;372(19):1832–1843. pubmed.ncbi.nlm.nih.gov
- Rosenblum JL. Fatigue in primary care. StatPearls. 2025. Fatigue accounts for 10 to 20% of primary care visits. ncbi.nlm.nih.gov